The key objective of this study is to collect the first nationally representative data on prevalence and correlates of DSM-IV MDD from the recently completed National Comorbidity Survey Replication (NCS-R).The study design is a direct interview household survey of a probability household survey of adults ages 18 and over from the 48 contiguous United States. The total number of subjects in the study is 9090, representing a 79% response rate. The diagnostic interview was the WHO Composite International Diagnostic Interview (CIDI), developed to collect diagnostic criteria for the DSM-IV. Clinical re-interviews were carried out with the Structured Clinical Interview for DSM-IV (SCID) to validate CIDI diagnoses. The NCS research program consists of a series of surveys associated with the U.S. National Comorbidity Survey (NCS). The baseline NCS, fielded from the fall of 1990 to the spring of 1992, was the first nationally representative mental health survey in the U.S. to use a fully structured research diagnostic interview to assess the prevalences and correlates of DSM-III-R disorders. The baseline NCS respondents are being reinterviewed in 2001-02 (NCS-2) to study patterns and predictors of the course of mental and substance use disorders and to evaluate the effects of primary mental disorders in predicting the onset and course of secondary substance disorders. In conjunction with this, an NCS Replication survey (NCS-R) is being carried out in a new national sample of 10,000 respondents. The goals of NCS-R are to study trends in a wide range of variables assessed in the baseline NCS and to obtain more information about a number of topics either not covered in the baseline NCS or covered in less depth than we currently desire. A survey of 10,000 adolescents (NCS-A) is being carried out in parallel with the NCS-R and NCS-2 surveys. The goal of NCS-A is to produce nationally representative data on the prevalences and correlates of mental disorders among youth. NCS-R and NCS-A, finally, are being replicated in a number of countries around the world. Centralized cross-national analysis of these surveys is being carried out by the NCS data analysis team under the auspices of the World Health Organization (WHO) World Mental Health Survey Initiative.The initial results indicate that 16.2% lifetime (32.6-35.1 million U.S. adults) and 6.6% 12-month (13.1-14.2 million U.S. adults). Sociodemographic correlates are female, younger than 60, unmarried, not black, unemployed, and disabled. Nearly three-fourths of MDD cases have comorbid anxiety, substance, or impulse control disorders. MDD is temporally primary in only a minority of these cases. Respondents with serious-severe 12-month MDD (38% of all 12-month MDD) averaged 83.5 days out of role, while mild-moderate cases averaged 9.6 days. Although a high proportion (72.4%) of serious-severe 12-month cases receive treatment, only a minority (36.9%) of treatment is adequate. Treatment adequacy is related to sector, but not severity. Sociodemographic correlates of severity, treatment, and treatment adequacy are far less numerous than the correlates of prevalence. In summary, we found that MDD is a common, seriously impairing, disorder that is widely distributed in the population. Early treatment of temporally primary disorders might be useful to prevent MDD. While the recent dramatic increase in treatment is encouraging, low treatment adequacy is a source of concern. The emphasis on screening, detection, and expansion of the number of MDD cases in treatment needs to be accompanied by a parallel emphasis on quality improvement. The NCS research program consists of a series of surveys associated with the U.S. National Comorbidity Survey (NCS). The baseline NCS, fielded from the fall of 1990 to the spring of 1992, was the first nationally representative mental health survey in the U.S. to use a fully structured research diagnostic interview too assess the prevalences and correlates of DSM-III-R disorders. The baseline NCS respondents are being reinterviewed in 2001-02 (NCS-2) to study patterns and predictors of the course of mental and substance use disorders and to evaluate the effects of primary mental disorders in predicting the onset and course of secondary substance disorders. In conjunction with this, an NCS Replication survey (NCS-R) is being carried out in a new national sample of 10,000 respondents. The goals of NCS-R are to study trends in a wide range of variables assessed in the baseline NCS and to obtain more information about a number of topics either not covered in the baseline NCS or covered in less depth than we currently desire. A survey of 10,000 adolescents (NCS-A) is being carried out in parallel with the NCS-R and NCS-2 surveys. The goal of NCS-A is to produce nationally representative data on the prevalences and correlates of mental disorders among youth. NCS-R and NCS-A, finally, are being replicated in a number of countries around the world. Centralized cross-national analysis of these surveys is being carried out by the NCS data analysis team under the auspices of the World Health Organization (WHO) World Mental Health Survey Initiative. During the past year we have completed writing the methodologic papers that were published in September, 2004. We have also begun to develop diagnostic algorithms for the disorders for which our NIMH team will have primary responsibility including mood spectrum disorders, headache, and smoking. We are also investigating the public health impact of the mental and physical disorders by calculating the magnitude of impairment that is attributable specifically to respective disorders. Publications from this study are beginning to emerge, and we expect to complete the several other key papers during the next year.